Its from the French term for extremely: "shesair"
The recent wave of colloquialisms within the French language have shortened it to shes.
However, modifying the French language in public forms is punishable by law so its become a young, underground movement.
Can be used in the same kind of context as "hella", but found more on the east coast.
"Let's get SH and watch Across the Universe. Good party!"